Set version to v0.15.1-dev

[chalin]

• Fixes Release 0.15.0 preparation #2501
• Closes https://github.com/google/docsy/milestone/23
• Bumps the repo dev line to 0.15.1-dev (including package.json and docsy.dev config tdVersion/params stamps across default, production, and doc-rooted configs).
• Adds an UNRELEASED changelog stub for v0.15.1 or v0.16.0 with placeholder sections and FIXME release links for later tagging.
• Hardens scripts/set-package-version: resolves the default config path to docsy.dev/.../params.yaml when present, otherwise ./hugo.yaml (docsy-example-style clones); updates tdVersion fields while preserving YAML structure and inline comments; when a scalar params.version exists, sets it to match dev (no leading v); expands tests accordingly.
• Fixes release-summary so the changelog anchor/label follows the blog-resolved release version, and uses {{ with $blogPage }} so missing blog pages (e.g. doc-rooted / no matching post) no longer error on .Title / .RelPermalink.
• Wires set:version:example / set:version:example:git-info to run the version script from a docsy-example sibling checkout (drops FIXME placeholders).
• Documents doc-rooted releases and refreshes maintainer-notes for the above workflow.
• Scope: internal-only (release tooling, docsy.dev layouts), docs-only (changelog template, maintainer guidance)